Predicting Movable Bed Roughness in Coastal Waters

Abstract

Accurately predicting movable bed roughness is essential to the analyses of transport processes, but when the bottom is rippled, as it commonly is in the coastal environment, characterizing the roughness is less straightforward than when the bottom is flat. The common method of predicting roughness, while effective, unnecessarily predicts ripple geometry and requires a model-dependent factor, which varies widely, relating ripple geometry and bottom roughness. We have therefore developed an alternative, more direct method of predicting bed roughness in the ripple regime: the wave energy dissipation factor is predicted from flow and sediment information and then any desired theoretical friction factor model is used to back-calculate the roughness. This paper describes the common and proposed methods of predicting roughness and presents results of preliminary testing of the methods with field data. Both methods adequately predict current velocities in wave-current field flows, with the proposed method yielding the smaller RMS-error of 3.1 cm/s. Remaining questions concerning the appropriate near-bottom orbital velocity required to describe field conditions must be resolved when additional field data becomes available.

A 3d Sediment Transport Model for Combined Wave-Current Flows

“…A wave-current interaction model (Grant and Madsen, 1979;Madsen, 1994;Humbyrd and Madsen, 2010) is incorporated into POM to account for wind wave effects. The model is based on a bilinear time-invariant eddy viscosity model that is proportional to the combined maximum wavecurrent shear velocity *m u within and to the enhanced current shear velocity *c u above the bottom boundary layer, i.e.…”

“…For rippled bed conditions, the physical roughness in (5) can be calculated iteratively through the empirical relationship for the energy dissipation factor established by Humbyrd and Madsen (2010) ( )…”